It is well known that scattered radiation impairs image quality, particularly in the case of imaging tomography scanners, such as e.g. in the case of computed tomography scanners. In order to reduce the detected proportion of the scattered radiation in the detector signals, so-called scattered-radiation collimators are arranged upstream of the radiation detectors in such computed tomography scanners.
By way of example, known scattered-radiation collimators comprise absorber elements, which are arranged next to one another in a collimation direction and are aligned unidirectionally in respect of their longitudinal extent. The absorber surfaces of the absorber elements are aligned radially in a fan-like shape with respect to the focus of a radiation source, and so only radiation from a spatial direction directed at the focus can impinge on the radiation detector. The radiation from this spatial direction is usually also referred to as primary radiation. Scattered radiation is substantially generated as a result of interaction processes between the primary radiation and the object to be examined. Scattered-radiation components that impinge on the scattered-radiation collimator from a different spatial direction than the primary radiation are mostly absorbed by the absorber surfaces. This affords a reduction in image artifacts caused by scattered-radiation effects in the reconstructed image.
The absorber elements of the scattered-radiation collimators are generally comparatively thin and delicate. As a result of this, they have, as such, low mechanical stability and are therefore not very dimensionally stable. Particularly when the recording system in a computed tomography scanner rotates, centrifugal forces and transverse forces perpendicular to the collimation direction are exerted on the absorber elements, causing deformations in the absorber elements and hence can lead to artifacts in the recorded attenuation values and hence in the reconstructed image, for example as a result of shadowing the detector elements.
In order to avoid deformations and temporary displacements, or in order to increase the stability of the absorber elements, two different approaches are substantially followed when a scattered-radiation collimator is implemented. The absorber elements of the scattered-radiation collimator are held in a support frame in the form of a plastics housing in a first approach. The plastics housing has very precise corresponding cut-outs on opposite sides of the support frame for a hold across the collimation direction. In this case, the scattered-radiation collimator is dimensioned such that it integrally spans the radiation detector in the z-direction. Such a scattered-radiation collimator is also referred to as a bridge collimator. However, a disadvantage of this embodiment is that, in particular, only very limited lengths of such a scattered-radiation collimator can be produced for injection-molding technical reasons, and so these scattered-radiation collimators can only be used in radiation detectors of a limited length or a restricting Z-coverage. However, radiation detectors are increasingly being designed with an ever increasing number of detector rows and hence with an increased coverage in the z-direction. Using bridge collimators in such radiation detectors is becoming evermore difficult for this reason.
In another approach, the scattered-radiation collimators are produced in small units and bonded onto the radiation detector in a tiled shape or like a matrix. Hence, scattered-radiation collimators with a comparatively large Z-coverage can be assembled, that is to say they can also be assembled for radiation detectors with a multiplicity of detector rows. Such a scattered-radiation collimator is also referred to as a tile collimator. However, a disadvantage of these scattered-radiation collimators is that, in particular, there is the risk of gaps forming at the seams that have to be present between adjacent scattered-radiation collimators as a result of the tile-shape design, through which gaps scattered radiation can impinge on the detector elements.
Moreover, in future there will also be higher mechanical demands on scattered-radiation collimators. Previously, rotational speeds of 210 rpm have been reached by the recording system in computed tomography scanners during examination operation. However, in future, the rotational speeds should be increased to at least 300 rpm. As a result of the higher centrifugal and transverse forces thereby exerted on the scattered-radiation collimator, the demands on dimensional stability are becoming evermore important.